<!doctype html>
<title>CSS Test Reference</title>
<style>
body {
  font-size: 30px;
}

.container {
  font-size: 5px;
  height: 40px;
}

.container::after {
  content: "";
  display: block;
  border: 2px solid blue;
  width: 1em;
  height: 1em;
  font-size: 1em;
}
</style>
<div class="container"></div>
<div class="container"></div>
<div class="container"></div>
<div class="container"></div>
<div class="container"></div>